Constant voltage power supply circuit

ABSTRACT

A disclosed constant voltage power supply circuit ( 1 ) has an input terminal (Vdd), an output terminal (Vout), a constant voltage power supply unit ( 2 ) that generates a constant voltage (Vo 1 ) with a ripple voltage (Vri), and a ripple removing circuit unit ( 3 ) for removing the ripple voltage so that a constant voltage (V 1 ) without the ripple voltage is output at the output terminal (OUT). The ripple removing circuit unit comprises a resistor (R 1 ) connected between the constant voltage power supply unit and the output terminal; a ripple voltage detection circuit unit ( 5 ) for detecting the ripple voltage and outputting a signal depending on the detected ripple voltage; and a current circuit unit ( 6, 7 ) for receiving the signal from the ripple voltage detection circuit unit and supplying a current (io 1 ) to the output terminal or absorbing a current (io 2 ) from the resistor in response to the received signal, so as to cancel the ripple voltage at the output terminal.

TECHNICAL FIELD

The present invention generally relates to a constant voltage powersupply circuit, and especially relates to such a constant voltage powersupply circuit that can cancel a ripple voltage while maintaining highvoltage efficiency.

BACKGROUND ART

In constant voltage power supply circuits including a constant voltagepower supply unit, a ripple voltage is inherently generated in an outputvoltage of the constant voltage power supply unit due to a variety ofcauses. A ripple filter is known for removing such ripple voltage asdisclosed in Patent Reference No. 1.

FIG. 5 shows a conventional constant voltage power supply circuit 100using a ripple filter.

The constant voltage power supply circuit 100 comprises a constantvoltage power supply unit 101 that generates and outputs a predeterminedconstant voltage, and a ripple filter 102 connected between an outputend A and an output terminal OUTa.

The ripple filter 102 comprises an NPN transistor Qa, a resistor Ra anda capacitor Ca. The NPN transistor Qa has a collector connected to theoutput end A of the constant voltage power supply unit 101, an emitterconnected to the output terminal OUTa, and a base connected to a nodebetween the resistor Ra and the capacitor Ca. Another end of thecapacitor Ca is grounded and another end of the resistor Ra is connectedto the output end A of the constant voltage power supply unit 101. Atime constant of the resistor Ra and the capacitor Ca is adjusted so asto be long enough compared with a frequency of the ripple voltage to beremoved.

In operation, when the ripple voltage rises, that is, when an outputvoltage Va of the constant voltage power supply unit 101 rises, acurrent flowing through the resistor Ra is increased and the increasedcurrent charges the capacitor Ca and raises a voltage across thecapacitor Ca. However, since the time constant of the resistor Ra andthe capacitor Ca is adjusted so as to be long enough compared with theripple frequency, the voltage across the capacitor Ca does not changesignificantly during the period of the ripple voltage rising. As aresult, a base voltage of the NPN transistor Qa is stable and thereforea voltage Voa at the output terminal OUTa does not change significantly.

On the other hand, when the ripple voltage falls down, that is, when theoutput voltage Va of the constant voltage power supply unit 101 islowered, the current flowing through the resistor Ra is decreased andthe capacitor Ca discharges. However, since the voltage across thecapacitor does not actually change significantly during the period ofthe ripple voltage falling, the base voltage of the NPN transistor Qa isstable, and therefore the voltage Voa at the output terminal OUTa doesnot change significantly.

FIG. 6 is a block diagram of another type of conventional constantvoltage power supply circuit. The constant voltage power supply circuitshown in FIG. 6 has a DC/DC converter 105, and a series regulator 106connected between an output end of the DC/DC converter and an outputterminal of the constant voltage power circuit. The DC/DC converter 105inherently generates a high frequency ripple voltage in its outputvoltage. The ripple voltage is removed by the series regulator 106.

[Patent reference 1] JPA 5-95628

The constant voltage power supply circuit shown in FIG. 5 generates alarge voltage drop between the output end A of the constant voltagepower supply unit 101 and the output terminal OUTa. The voltage of thecapacitor Ca must be higher than the output voltage Voa by abase-emitter voltage Vbe for the NPN transistor Qa. The capacitor Ca ischarged through the resistor Ra and therefore the output voltage Va ofthe constant voltage power supply unit 101 must be high, and thereforepower supply efficiency is degraded.

In order to make the time constant of the resistor Ra and the capacitorCa large enough, either a resistance value of the resistor Ra or acapacitance of the capacitor Ca must be made large.

If the resistance value of the resistor Ra is made large, a base currentof the NPN transistor Qa is reduced. It is not desired to reduce thebase current of the NPN transistor Qa, because all the current suppliedto the output terminal OUTa and a load (not shown) connected theretoflows through the NPN transistor Qa. Therefore, if the resistance valueof the resistor Ra becomes large, the output voltage Va of the constantvoltage power supply unit 101 should be larger in order to give enoughbase current of the NPN transistor Qa. As a result, the voltage dropacross the NPN transistor Qa is further increased, and therefore thepower supply efficiency is degraded.

On the other hand, if the capacitance of the capacitor Ca is made large,the large size of the capacitance Ca makes it impossible to integratethe capacitor Ca and the capacitor Ca has to be externally attached.

Also in the constant voltage power supply circuit shown in FIG. 6, theseries regulator 106 has a voltage drop. When an output current of theseries regulator 106 is large, the voltage drop thereof also becomeslarge, resulting in degradation of the power supply efficiency.

DISCLOSURE OF THE INVENTION

It is a general object of the present invention to provide a constantvoltage power supply circuit that can cancel a ripple voltage whilemaintaining high voltage efficiency.

Features and advantages of the present invention are set forth in thedescription that follows, and in part will become apparent from thedescription and the accompanying drawings, or may be learned by practiceof the invention according to the teachings provided in the description.Objects as well as other features and advantages of the presentinvention will be realized and attained by an information recordingapparatus and a method thereof particularly pointed out in thespecification in such full, clear, concise, and exact terms as to enablea person having ordinary skill in the art to practice the invention.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides as follows.

According to one aspect of the present invention, in a constant voltagepower supply circuit having an input terminal, an output terminal, aconstant voltage power supply unit that generates a constant voltagewith a ripple voltage, and a ripple removing circuit unit for removingthe ripple voltage so that a constant voltage without the ripple voltageis output at the output terminal,

the ripple removing circuit unit comprises:

a resistor connected between the constant voltage power supply unit andthe output terminal;

a ripple voltage detection circuit unit for detecting the ripple voltageand outputting a signal depending on the detected ripple voltage; and

a current circuit unit for receiving a signal from the ripple voltagedetection circuit unit and supplying a current to the output terminal orabsorbing a current from the resistor in response to the receivedsignal, so as to cancel the ripple voltage at the output terminal.

The current circuit unit may supply a current when the received signalindicates that the ripple voltage is negative, and may absorb a currentwhen the received signal indicates that the ripple voltage is positive.

The current circuit unit may comprise a current supplying circuit unitand a current absorbing circuit unit.

The current supplying circuit unit may vary the supplied currentdepending on the received signal, and the current absorbing circuit unitmay vary the absorbed current depending on the received signal.

The product of a resistance value of the resistor and the supplied orabsorbed current may be equal to the ripple voltage.

The ripple removing circuit unit may further comprise a capacitorconnected between the output terminal and an output of the currentcircuit unit.

The ripple voltage detection circuit unit may receive the voltage fromthe constant voltage power supply unit, compare the received voltagewith a reference voltage, and output the signal in accordance with thecomparison.

The constant voltage power supply unit and the ripple removing circuitunit may be integrated into one IC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a constant voltage power supply circuitaccording to an embodiment of the present invention.

FIG. 2 shows a current io1 supplied by a current supplying circuit and acurrent io2 absorbed by a current absorbing circuit in response to aripple voltage Vri.

FIG. 3 shows an example circuit of a ripple removing circuit unit 3shown in FIG. 1.

FIG. 4 shows another example circuit of the ripple removing circuit unit3 shown in FIG. 1.

FIG. 5 shows a conventional constant voltage power supply circuit usinga ripple filter.

FIG. 6 shows another type of conventional constant voltage power supplycircuit.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, an embodiment of the present invention is describedwith reference to the accompanying drawings.

The embodiment of the present invention is explained with reference toFIG. 1 through FIG. 4. FIG. 1 is a block diagram of a constant voltagepower supply circuit 1 according to the embodiment of the presentinvention.

The constant voltage power supply circuit 1 comprises a constant voltagepower supply unit 2 that receives a power supply voltage Vdd and isideally desired to generate and output a predetermined constant voltageV1, and a ripple removing circuit unit 3 connected between an output endOUT1 of the constant voltage power supply unit 2 and an output terminalOUT of the constant voltage power supply circuit 1.

The ripple removing circuit unit 3 comprises a resistor R1 connectedbetween the output end OUT1 of the constant voltage power supply unit 2and the output terminal OUT, a ripple voltage detection circuit 5 fordetecting a ripple voltage Vri appearing in an actual output voltage Vo1of the constant voltage power supply unit 2, a current supplying unit 6for supplying a current io1 to the output terminal OUT in accordancewith an output signal from the ripple voltage detection circuit 5, and acurrent absorbing circuit 7 for absorbing a current io2 via the resistorR1 in accordance with the output signal from the ripple voltagedetection circuit 5. The ripple voltage detection circuit 5 is anexample of a ripple voltage detection circuit unit, the currentsupplying circuit 6 is an example of a current supplying circuit unit,and the current absorbing circuit 7 is an example of a current absorbingcircuit unit. The combination of the current supplying circuit unit andthe current absorbing circuit unit is an example of a current circuitunit. It is desirable that the constant voltage power supply unit andthe ripple removing circuit unit be integrated into one IC.

To an input end of the ripple voltage detection circuit 5, the outputvoltage Vo1 of the constant voltage power supply unit 2 is input. Anoutput signal of the ripple voltage detection circuit 5 is input to thecurrent supplying circuit 6 and the current absorbing circuit 7. Thecurrent io1 supplied from the current supplying circuit 6 is output tothe output terminal OUT. The current io2 absorbed by the currentabsorbing circuit 7 is input via the resistor R1 to the currentabsorbing circuit 7.

In this structure, the ripple removing circuit unit 3 operates in amanner shown in FIG. 2. FIG. 2 illustrates the ripple voltage Vrisuperposed on the constant voltage V1 output from the constant voltagepower supply unit 2, the output current io1 supplied from the currentsupplying circuit 6 in response to the ripple voltage Vri, and theoutput current io2 absorbed by the current absorbing circuit 7 inresponse to the ripple voltage Vri.

In FIG. 2, when the ripple voltage Vri is negative, that is, when theoutput voltage Vo1 of the constant voltage power supply unit 2 is lowerthan the constant voltage V1, the current absorbing circuit 7 stops itsoperation and the current supplying circuit 6 operates. The current io1supplied from the current supplying circuit 6 varies according to theripple voltage Vri, and becomes larger as the ripple voltage Vri becomeslower. When the supplied current io1 becomes larger, a current flowingthrough the resistor R1 is reduced and a voltage drop over the resistorR1 is also reduced, and that prevents an output voltage Vout at theoutput terminal OUT from falling down.

A resistance amount of the resistor R1 and/or an amount of the suppliedcurrent io1 from the current supplying circuit 6 can be adjusted so thatthe reduction in the voltage drop across the resistor R1 due to thesupplied current io1 is equal to the ripple voltage decrease from theconstant voltage V1. In this manner, it is possible to prevent theoutput voltage Vout at the output terminal OUT from falling down.

Next, when the ripple voltage Vri is positive, that is, when the outputvoltage Vo1 of the constant voltage power supply unit 2 is higher thanthe constant voltage V1, the current supplying circuit 6 stops itsoperation and the current absorbing circuit 6 operates. The current io2absorbed by the current supplying circuit 7 varies according to theripple voltage Vri, and becomes larger as the ripple voltage Vri becomeshigher. When the absorbed current io2 becomes larger, a current flowingthrough the resistor R1 increases and a voltage drop across the resistorR1 also increases, and that prevents the output voltage Vout at theoutput terminal OUT from rising up.

A resistance amount of the resistor R1 and/or an amount of the absorbedcurrent io2 by the current absorbing circuit 6 can be adjusted so thatthe increment in the voltage drop across the resistor R1 due to theabsorbed current io2 is equal to the ripple voltage increment from theconstant voltage V1. In this manner, it is possible to prevent theoutput voltage Vout at the output terminal OUT from rising up.

FIG. 3 shows a circuit example of the ripple removing circuit unit 3.The ripple voltage detection circuit 5 comprises a reference voltagegenerating circuit 11 for generating a predetermined voltage Vs1, acapacitor C2, and two pairs of resistors R4, R5 and R6, R7. The currentsupplying circuit 6 comprises an operational amplifier AMP1, a PMOStransistor M1, a capacitor C1, and resistors R2, R8 and R9. The currentabsorbing circuit 7 comprises an operational amplifier AMP2, a NMOStransistor M2, the capacitor C1, and resistors R3, R10 and R11. Thecapacitor C1 is an example of a first capacitor, the capacitor C2 is anexample of a second capacitor, the pair of resistors R4 and R5 is anexample of a first voltage dividing circuit, and the pair of resistorsR6 and R7 is an example of a second voltage dividing circuit.

Between the reference voltage Vs1 and the ground voltage, the seriallycoupled resistors R4 and R5 and the serially coupled resistors R6 and R7are connected in parallel.

A node between the resistor R4 and the resistor R5 is coupled to theoutput end OUT1 of the constant voltage power supply unit 2 via thecapacitor C2. The node between the resistor R4 and the resistor R5 isfurther coupled via the resistor R8 to a non-inverting input of theoperational amplifier AMP1 and coupled via the resistor R10 to anon-inverting input of the operational amplifier AMP2.

A node between the resistor R6 and the resistor R7 is coupled toinverting inputs of the operational amplifiers AMP1 and AMP2.

To an output end of the operational amplifier AMP1, a gate of the PMOStransistor M1 is connected. To an output end of the operationalamplifier AMP2, a gate of the NMOS transistor M2 is connected. Betweenthe power supply voltage Vdd and the ground voltage, the PMOS transistorM1, the resistor R2, the resistor R3 and the NMOS transistor M2 areconnected in series. A node between the resistor R2 and the resistor R3is coupled to the output terminal OUT via the capacitor C1. Thecapacitor C1 is for cutting a direct current voltage.

A node between a drain of the PMOS transistor M1 and the resistor R2 isconnected via the resistor R9 to the non-inverting input of theoperational amplifier AMP1. A node between the resistor R3 and the NMOStransistor M2 is connected via the resistor R11 to the non-invertinginput of the operational amplifier AMP2.

In this structure, the current supplying circuit 6 has an invertingamplifying circuit formed by the operational amplifier AMP1 and the PMOStransistor M1, and its amplification factor is generally represented bythe resistance value of the resistor R9 divided by the resistance valueof the resistor R8. Similarly, the current absorbing circuit 7 has aninverting amplifying circuit formed by the operational amplifier AMP2and the NMOS transistor M2, and its amplification factor is generallyrepresented by the resistance value of the resistor R11 divided by theresistance value of the resistor R10.

The resistance amounts of the resistors R4˜R7 are adjusted so that aratio of the resistor R4 and the resistor R5 is equal to a ratio of theresistor R6 and the resistor R7. In this manner, when there is no ripplevoltage, a voltage V2 at the node between the resistor R4 and theresistor R5 is equal to a voltage V3 at the node between the resistor R6and the resistor R7.

When a ripple voltage appears in the output voltage Vo1 of the constantvoltage power supply unit 2, the ripple voltage varies the voltage V2 atthe node between the resistor R4 and the resistor R5 through thecapacitor C2. On the other hand, the voltage V3 at the node between theresistor R6 and the resistor R7 is not varied. Therefore, a voltagedifference between the voltages V2 and V3 is input to the currentsupplying circuit 6 and the current absorbing circuit 7 and amplifiedtherein, and appears at the drains of the PMOS transistor M1 and theNMOS transistor M2.

When the voltage V2 is lower than the voltage V3, that is, when theripple voltage Vri is negative, the voltage difference is amplified bythe current supplying circuit 6 and the current absorbing circuit 7, andthe amplified voltages turn on the PMOS transistor M1 and turn off theNMOS transistor M2, respectively, and therefore raise the drain voltageof the PMOS transistor M1 and the source voltage of the NMOS transistorM2, and raise the voltage at the node between the resistors R2 and R3.The raised voltage is applied via the capacitor C1 to the outputterminal OUT and prevents the voltage at the output terminal OUT fromfalling down. The current io1 supplied from the current supplyingcircuit 6 to the output terminal OUT is determined by the amplificationfactor of the current supplying circuit 6 and the resistance value ofthe resistor R2. As mentioned with reference to FIGS. 1 and 2, theripple voltage Vri can be cancelled at the output terminal OUT byadjusting so that the product of the current io1 and the resistor R1 isequal to the ripple voltage Vri.

On the other hand, when the voltage V2 is higher than the voltage V3,that is, when the ripple voltage Vri is positive, the voltage differenceis amplified by the current supplying circuit 6 and the currentabsorbing circuit 7, and the amplified voltages turn off the PMOStransistor M1 and turn on the NMOS transistor M2, respectively, andtherefore lower the drain voltage of the PMOS transistor M1 and thesource voltage of the NMOS transistor M2, and lower the voltage at thenode between the resistors R2 and R3. The lowered voltage is applied viathe capacitor C1 to the output terminal OUT and prevents the voltage atthe output terminal OUT from rising up. The current io2 absorbed by thecurrent absorbing circuit 7 from the resistor R1 is determined by theamplification factor of the current absorbing circuit 7 and theresistance value of the resistor R3. As mentioned with reference toFIGS. 1 and 2, the ripple voltage Vri can be cancelled at the outputterminal OUT by adjusting so that the product of the current io2 and theresistor R1 is equal to the ripple voltage Vri. Since the resistorsR8˜R11 are larger than resistors R2 and R3 enough, the currents suppliedand absorbed through the resistors R2 and R3 to and from the outputterminal OUT are not substantially affected.

In FIG. 3, the current supplying circuit 6 and the current absorbingcircuit 7 are formed by the two operational amplifiers AMP1 and AMP2,respectively. However, the current supplying circuit and the currentabsorbing circuit can be formed by one operational amplifier AMP1 asshown in FIG. 4.

Members shown in FIG. 4 that are the same as or similar to the membersshown in FIG. 3 are assigned the same or similar alphanumeric referencesand their explanations are omitted. Only points different from FIG. 3are explained below.

In an example shown in FIG. 4, compared with FIG. 3, the operationalamplifier AMP2, the PMOS transistor M1, the NMOS transistor M2 and theresistors R10, R11 are removed, and diodes D1 and D2 are added.

A current supplying circuit 6 shown in FIG. 4 comprises an operationalamplifier AMP1, the diode D1, a capacitor C1 and resistors R2, R8 andR9. A current absorbing circuit 7 shown in FIG. 4 comprises theoperational amplifier AMP1, the diode D2, the capacitor C1 and resistorsR3, R8 and R9.

A node between the resistor R4 and the resistor R5 is coupled via theresistor R8 to an inverting input of the operational amplifier AMP1. Anode between the resistor R6 and the resistor R7 is coupled to anon-inverting input of the operational circuit AMP1. The resistor R9 isconnected between the non-inverting input and an output of theoperational amplifying circuit AMP1. One end of the capacitor C1 isconnected to the output terminal OUT. Between another end of thecapacitor C1 and the output end of the operational amplifying circuitAMP1, serially connected diode D1 and resistor R2 and serially connecteddiode D2 and resistor R3 are coupled in parallel.

In operation, the ripple voltage Vri is amplified by an invertingamplifying circuit formed by the operational amplifying circuit AMP1 andthe resistors R8, R9. When the ripple voltage Vri is negative, theoutput voltage of the operational amplifying circuit AMP1 is raised andthe operational amplifying circuit AMP1 supplies a current io1 throughthe diode D1 and the resistor R2 to the output terminal OUT. When theripple voltage Vri is positive, the output voltage of the operationalamplifying circuit AMP1 falls down and the operational amplifyingcircuit AMP1 absorbs a current io2 through the diode D2 and the resistorR3 from the resistor R1. If the current supplied to the output terminalOUT and the current absorbed from the resistor R1 are equal to eachother, the diodes D1 and D2 can be omitted and the resistors R2 and R3can be combined into one resistor, which is connected between theoperational amplifying circuit AMP1 and the capacitor C1.

In the examples shown in FIGS. 3 and 4, the current supplying circuit 6and the current absorbing circuit 7 share one capacitor C1 in common.Alternatively, each of the current supplying circuit 6 and the currentabsorbing circuit 7 may be provided with a capacitor C1, each of whichis connected between the resistor R2 and the output terminal OUT andbetween the resistor R3 and the output terminal OUT, respectively.

In the constant voltage power supply circuit according to the embodimentof the present invention, when the ripple voltage Vri is negative, thatis, when the output voltage Vo1 is lower than the constant voltage V1,the current absorbing circuit 7 stops its operation and the currentsupplying circuit 6 operates. An amount of the current io1 supplied bythe current supplying circuit 6 becomes larger as the ripple voltage Vriis smaller. When the ripple voltage Vri is positive, that is, when theoutput voltage Vo1 is higher than the average voltage V1, the currentsupplying circuit 6 stops its operation and the current absorbingcircuit 7 operates. An amount of the current io2 absorbed by the currentabsorbing circuit 7 becomes larger as the ripple voltage Vri is larger.Accordingly, the ripple voltage of the output voltage Vo1 of theconstant voltage power supply unit 2 can be cancelled. The voltage dropbetween the output end OUT1 of the constant voltage power supply unit 2and the output terminal OUT of the constant voltage power supply circuit1 can be reduced, and therefore power supplying efficiency can beimproved when outputting a large current.

Further, the present invention is not limited to the above embodiment,but variations and modifications may be made without departing from thescope of the present invention.

The present application is based on Japanese Priority Application No.2004-308369 filed on Oct. 22, 2004 with the Japanese Patent Office, theentire contents of which are hereby incorporated by reference.

1-8. (canceled)
 9. A constant voltage power supply circuit comprising: aconstant voltage power supply unit configured to generates a constantvoltage; and a ripple removing circuit unit for removing a ripplevoltage in said constant voltage generated by said a constant voltagepower supply unit, so that an output voltage without the ripple voltageis output at an output terminal.
 10. The constant voltage power supplycircuit of claim 9, wherein the ripple removing circuit unit includes aresistor connected between the constant voltage power supply unit andthe output terminal; a ripple voltage detection circuit unit fordetecting the ripple voltage and outputting an output signal dependingon the detected ripple voltage; and a current circuit unit configured toreceive said output signal from the ripple voltage detection circuitunit and supplying a current to the output terminal in response to thereceived signal.
 11. The constant voltage power supply circuit of claim10, wherein the current supplied to the output terminal cancels theripple voltage at the output terminal.
 12. The constant voltage powersupply circuit of claim 9, wherein the ripple removing circuit unitincludes a resistor connected between the constant voltage power supplyunit and the output terminal; a ripple voltage detection circuit unitfor detecting the ripple voltage and outputting an output signaldepending on the detected ripple voltage; and a current circuit unitconfigured to receive said output signal from the ripple voltagedetection circuit unit and absorb a current from the resistor inresponse to the received signal.